In recent past, nanoparticles or quantum dots, nanowires, nanorods have become attractive because of the properties that are different from their bulk counterparts. Their unique characteristics may be attributed to quantum confinement effects, which may have led to band gap engineering. Zinc Sulfide (ZnS), a wide band gap semiconductor, has found various applications as optical phosphors, photonic crystals, UV sensors, photo-conducting switch, solar cells, field emitters, and light emitting materials. ZnS absorbs only UV radiation because of its large band gap, but its absorption and emission band may be altered by doping it with metal ions, such as manganese (Mn), nickel (Ni), copper (Cu), and lead (Pb). Furthermore, intrinsic defects, such as Zn and S electronic vacancies in ZnS nanostructures, may play a role in the visible emission. The amount of Zn and S vacancies has been reported to depend strongly on the growth and post growth processing conditions.
In general, ZnS in its Wurtzite phase may be more desirable than in its Sphalerite phase because of superior optical properties in its Wurtzite phase. ZnS nanowires and nanobelts have been demonstrated to have similar photoluminescence (PL) emission bands. Incorporation of metal ions actually offers an efficient radiative channel by introducing a band gap state, which may alter the optical properties of the host material. Thus, the choice and location of the metal ion in the host lattice is very important in defining the radiative recombination pathway.
Photoluminescence and electroluminescence properties of Mn doped ZnS have been extensively studied. ZnS has attained unusual attention in electroluminescent devices, lasers, and flat panel displays when doped with Mn. Doping of Mn in ZnS crystal has been reported to reduce the probability of nonradiative recombinations, and to make the Mn doped ZnS to phosphor in the range 590 nm to 620 nm due to radiative recombinations between the Mn d-states. Coupling of sp states of nanocrystalline ZnS and 3d states of Mn may result in faster transfer of electron from ZnS band to Mn 3d states in 5 orders of magnitude. The PL spectrums of undoped ZnS nanobelts and Mn doped ZnS nanobelts have shown some emission bands at 440 nm and 540 nm, bands of which may be assigned to defects produced by Sulfur (S) and Zn electron vacancies, respectively, while the emission band observed at 590 nm Mn doped ZnS may be attributed to Mn d-d state transition. In the case of Cu-doped ZnS nanorods, green emission may be attributed to elemental S species present on a nanorod surface, and orange emission may be associated with recombination of electron at deep defect levels of Cu introduced states. Surface defect states are well known to be responsible for the blue emission at 410 nm, green emission at 540 nm originated from Zn vacancy states, or interstitial states.
Recently, the role of the intrinsic defects like Zn and S interstitials and vacancies on the visible luminescence from the ZnS nanostructure has been studied. The interstitials and vacancies created by both Zn and S may lead to strong luminescence in the band gap with varying decay times. The magnitude of defects may be controlled by annealing the nanostructures in an inert atmosphere, in which the annealing may affect the optical properties.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the inventive concept, and, therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.